Séminaire – Kuldeep Ray

Mercredi 1er juillet à 11h00 (auditorium TRT), nous accueillerons Dr. Kuldeep RAY (Laboratoire Albert Fert, France) pour un séminaire.

Title:
“Towards Reliable SOT-MRAM Technology for Functional Devices”.

Abstract :

With the exponential growth of microelectronics and the increasing number of smartphones and edge devices, the amount of data processed and stored has also skyrocketed. In addition, the extreme scaling of transistor technology to the sub-10nm regime has significantly increased the power consumption of such devices. The use of non-volatile memory closer to the central processing unit (CPU) could minimize static power consumption and improve energy efficiency. Magnetic random-access memory (MRAM) is one of the leading candidates to bring non-volatility closer to the CPU. In the latest generations of MRAM, the free layer can be switched using current-induced torques. Spin-transfer torque MRAM (STT-MRAM) uses a spin-polarized current through the MTJ to switch the free layer and is already in large-scale production. Spin-orbit torque MRAM (SOT-MRAM), on the other hand, uses an in-plane charge current to generate an out-of-plane spin current by the spin Hall and Rashba effects and manipulate the free layer magnetization. SOT-MRAM is particularly interesting due to its fast sub-nanosecond switching times and superior endurance. However, several challenges need to be addressed before commercialization.

First, the challenge of achieving deterministic switching over a wide current range is considered. Although SOT-MRAM writing under the application of an external magnetic field is considered to be deterministic, the writing probability for SOT-MRAM has been reported to reduce from 1 at high currents. This loss of determinism arises from an intrinsic backswitching mechanism. Here, we present a comprehensive study of the backswitching in sub-100nm CoFeB magnetic pillars on β-W Hall crosses. The impact of applied SOT pulse parameters and applied magnetic fields in different directions is discussed using a statistical approach. Macrospin simulations, using realistic magnetic parameters, reproduce the experimental observations and provide insights into the backswitching mechanism. Based on these simulations, we propose several strategies to reduce backswitching, including tailoring the SOT pulse shapes.  [1]

Next, we address the requirement of an external magnetic field for deterministic switching. A more practical approach combines SOT and STT effects. However, the use of STT in SOT-MRAM introduces reliability issues typical of STT-MRAM. We highlight how imperfections in the reference layer affect switching under combined SOT/STT using write error rate (WER) measurements and macrospin simulations. Experimentally, we demonstrate that modified pulse shapes can improve WER [2]. We also characterize switching in SOT-MRAM devices with an improved top stack, systematically studying how pulse width, amplitude, and timing influence WER. Using optimized pulsing parameters, we achieve a deep WER under sub-critical SOT and STT currents. Furthermore, we discuss how stray fields contribute to switching asymmetry. Macrospin simulations further elucidate the interplay of SOT and STT during reversal.

Finally, we explore emerging orbital torques for efficient switching. In a Co/Pt/Ta trilayer, we measure both SOT and orbital torque efficiencies as a function of temperature, finding that the damping-like torque is more than twice that of Pt/Co alone and increases at higher temperatures, alongside a pronounced field-like component. These results uncover rich physical phenomena and point toward solutions for key SOT-MRAM challenges.

REFERENCES

[1]          K. Ray, J. Vigier, P. Usé, S. Martin, N. Lefoulon, C. Bouard, M. Drouard, and G. Gaudin, Intrinsic back-switching phenomenon in spin-orbit torque MRAM devices, Phys. Rev. Applied 24, 064038 (2025).

[2]          K. Ray, J. Vigier, S. Martin, C. Bouard, N. Lefoulon, M. Drouard, and G. Gaudin, Pulse Shaping to Mitigate the Impact of Device Imperfections in Field-Free Switching Using Combined Spin-Orbit and Spin-Transfer Torques, IEEE Magnetics Letters 17, 4500305 (2026).